Negative Activation Energy Research Articles

Features of activation energy for a flow with microrotation near stagnation region and unbounded domain : a spectral simulation

This article is motivated to find the impact of various flow controlling parameters such as viscous dissipation, negative and positive values of activation energy on MHD stagnation point micropolar nanofluid flow over a surface. However, negative activation energy is scarce in practice, but the impact of negative values of activation energy could not be neglected as seen in chemical processes. The present mathematical model is solved with successive linearization method (a spectral technique). The velocity, angular velocity, temperature and concentration profiles are drawn by varying flow controlling parameters. The bar diagrams for skin friction, Nusselt number, Sherwood number and couple stress are also displayed. Results indicate that the activation energy boosts the concentration profile up. The negative values of activation energy have higher impact on concentration curves compared to positive values. Arrhenius function decays as activation energy enlarges. This eventually promotes the generative chemical reaction due to which concentration rises. By varying thermophoretic parameter from 0.1 to 0.3, 88.65% and 8.34% enhancement is captured in heat transfer and mass transfer respectively.

Flow-induced surface instabilities in a flexible-walled channel with a heavy wall

We consider the stability of laminar high-Reynolds-number flow through a planar channel formed by a rigid wall and a heavy compliant wall under longitudinal tension with motion resisted by structural damping. Numerical simulations indicate that the baseline state (with Poiseuille flow and a flat wall) exhibits two unstable normal modes: the Tollmien–Schlichting (TS) mode and a surface-based mode which manifests as one of two flow-induced surface instabilities (FISI), known as travelling wave flutter (TWF) and static divergence (SD), respectively. In the absence of wall damping the system is unstable to TWF, where the neutrally stable wavelength becomes shorter as the wall mass increases. With wall damping, TWF is restricted to long wavelengths through interaction with the most unstable centre mode, while for wall damping greater than a critical value the system exhibits an SD mode with a two branch neutral stability curve; the critical conditions along the upper and lower branches are constructed in the limit of large wall damping. We compute the Reynolds–Orr and activation energy descriptions of these neutrally stable FISI by continuing the linear stability analysis to the following order in perturbation amplitude. We find that both FISI are primarily driven by the working of normal stress on the flexible wall, lower-branch SD has negative activation energy, while upper-branch SD approaches zero activation energy in the limit of large wall damping. Finally, we elucidate interaction between TS and TWF modes for large wall mass, resulting in stable islands within unstable regions of parameter space.

Temperature‐dependent kinetics of the reactions of CH2OO with acetone, biacetyl, and acetylacetone

AbstractTemperature‐dependent rate constants for the reactions of CH2OO with acetone (Ac), biacetyl (BiAc), and acetylacetone (AcAc) have been measured over the range 275–335 K using a flash photolysis, transient absorption spectroscopy technique. The measurements were performed at a total pressure of ∼80 Torr in N2 bath gas, which corresponds to the high‐pressure limit for these reactions. All three reactions show linear Arrhenius plots with negative temperature dependences. Rate constants increase in the order Ac < AcAc « BiAc across the temperature range; at 295 K the rate constants are kAc = (4.8 ± 0.4) × 10–13 cm3 s–1, kAcAc = (8.0 ± 0.7) × 10–13 cm3 s–1, and kBiAc = (1.10 ± 0.09) × 10–11 cm3 s–1. Sensitivity to temperature, characterized by the magnitude of the negative activation energy, increases in the order AcAc < BiAc < Ac (Ea/R values of –1830 ± 170 K, –1260 ± 170 K, and –460 ± 180 K, respectively). CBS‐QB3 calculations show that the Ac and BiAc reactions proceed via formation of an entrance channel complex followed by 1,3‐dipolar cycloaddition to form secondary ozonide products via a submerged transition state. For the BiAc reaction, the rate limiting step appears to be rearrangement of a long‐range van der Waals complex into the short‐range complex that subsequently leads directly to the cycloaddition transition state with a very low energy barrier. The calculations show that two reaction pathways are competitive for AcAc with nearly identical transition state free energies (ΔG° = +10.1 kcal mol–1 at 298 K) found for cycloaddition at the C=O and at the C=C site of the dominant enolone tautomer. The weak temperature dependence observed is likely due to competition between these pathways.

Mechanistic Insights on the Low-Temperature Oxidation of CO Catalyzed by Isolated Co Ions in N-Doped Carbon

Isolated cobalt ions on nitrogen-doped carbon (Co–N–C) catalyze CO oxidation at temperatures as low as 196 K, but the active site and mechanism for this reaction remain elusive. In this work, steady-state CO oxidation around 273 K over Co–N–C revealed nearly first-order behavior in both CO and O2 as well as a negative apparent activation energy. Isotopic transient analysis of the reaction confirmed a rapid turnover frequency and low surface coverage of adsorbed intermediates leading to CO2 (<10% of the Co). Results from kinetics experiments combined with quantum chemical calculations and molecular dynamics simulations are consistent with a reaction path involving weak adsorption of CO onto Co ions followed by a low barrier for the CO-assisted activation of weakly adsorbed O2. This proposed mechanism for dioxygen activation does not involve a redox cycle with the transition-metal ion and may be important in other low-temperature catalytic reactions involving O2.

Surface protonic conductivity in chemisorbed water in porous nanoscopic CeO2

CeO2 surfaces play decisive roles in heterogeneous catalysis of important processes. Here, we investigate adsorption and dissociation of water and migration of protons on internal surfaces of nanoscopic porous CeO2. Sorption and thermogravimetry confirm literature suggestions that the surface is hydrogenated to Ce3+ ions and protons H+. The following chemisorption is dissociative, yet weak, and physisorption sets in only at the very highest relative humidities, reflecting hydrophobic behaviour. We link sample conductivities to surface protonic conductances via a brick layer model and show that behaviours at high, intermediate, and low temperatures with, respectively, positive, close to zero, and negative apparent activation energies and pH2O1/2, pH2O1, and pH2O3/2 dependences, can be attributed to different models of migration all within the chemisorbed layer, without contribution from physisorbed water. While CeO2 may special in this respect due to the effect of the hydrogenated surface, we believe the extended models of transport in the chemisorbed layer may apply also to other oxides. Unsaturated chemisorption may play an important role for CeO2 as catalyst in that the surface is left available for reactant molecules, still with availability of dissociated and mobile protons in the chemisorbed layer and electronic defects by Ce3+ in the surface.

Measuring RNA UNCG Tetraloop Refolding Dynamics Using Temperature-Jump/Drop Infrared Spectroscopy.

Determining the structural dynamics of RNA and DNA is essential to understanding their cellular function, but direct measurement of strand association or folding remains experimentally challenging. Here we illustrate a temperature-jump/drop method able to reveal refolding dynamics. Time-resolved temperature-jump/drop infrared spectroscopy is used to measure the melting and refolding dynamics of a 12-nucleotide RNA sequence comprising a UACG tetraloop and a four-base-pair double-stranded GC stem, comparing them to an equivalent DNA (TACG) sequence. Stem-loop melting occurred an order of magnitude more slowly in RNA than DNA (6.0 ± 0.1 μs versus 0.8 ± 0.1 μs at 70 °C). In contrast, the refolding dynamics of both sequences occurred on similar time scales (200 μs). While the melting and refolding dynamics of RNA and DNA hairpins both followed Arrhenius temperature dependences, refolding was characterized by an apparent negative activation energy, consistent with a mechanism involving multiple misfolded intermediates prior to zipping of the stem base pairs.

Layer-structured hydrogen permeable membranes and their application in hydrogen membrane fuel cells

• Significantly improved hydrogen permeability at low temperature. • Negative activation energy for hydrogen permeation due to the V/Nb mid-layer. • Superior hydrogen permeability over the permeability of PdAg 25 at 298K. • Low-temperature operation of hydrogen membrane fuel cells. V and Nb are hydrogen permeable metals exhibiting greater hydrogen purification ability than Pd, but the permeability is not high enough due to low hydrogen dissociation and surface oxidation. In this study, two types of non-Pd hydrogen permeable membranes with a triple layer structure, Ni 64 Zr 36 /V/Ni 64 Zr 36 and Ni 64 Zr 36 /Nb/Ni 64 Zr 36 , have been prepared to produce a stable membrane exhibiting superior hydrogen permeability at low temperature range near to room temperature. The triple layer consists of 10 to 100 nm - thick amorphous Ni 64 Zr 36 thin films deposited by magnetron sputtering onto the surfaces of each 25 μm - thick crystalline V and Nb membranes. Significantly high hydrogen permeability over that of PdAg 25 below 373 K was observed in the prepared membranes. The performance of the triple membranes and the effect of the Ni 64 Zr 36 coating thickness on its hydrogen permeability are reported. Low-temperature hydrogen membrane fuel cells (LT-HMFCs) are built using the composite electrolytes composed of the suggested triple membrane and a glass fiber layer with phosphoric acid. The Nb and V interlayer plays an important role in implementing LT-HMFCs by improving hydrogen solubility and increasing hydrogen permeability at low temperature through its intrinsic activation energy, which is inversely proportional to temperature. The maximum output power obtained from LT-HMFC with 50 nm - thick V mid layer was 0.98 mW/cm 2 , at 423 K.

Sintering behavior of the Ti(C,N)‐based cermets with graphite or diamond additives

AbstractCombining DSC/TG–QMS analysis and dilatometry experiments at constant heating rates, the sintering behaviors of the Ti(C,N)‐based cermets with carbon additives were studied. It is found that the additives of diamond or graphite could promote the interactions among raw powders during solid‐state sintering. The outgassing behavior and endothermic effect were enhanced, thereby resulting in the increasing densification activation energy. The negative activation energy during the liquid phase sintering indicates that the dissolution–precipitation process occurs easily and provides a rapid densification path. The dissolution–precipitation process for cermets with .6‐wt% graphite or .6‐wt% diamond additive became more efficient than for cermets without additional carbon additive. The white‐core/gray‐rim grains were clustered together in the cermets with .6‐wt% graphite additive, whereas they were distributed evenly in the cermets with .6‐wt% diamond additive. Moreover, the average sizes of ceramic grains in the cermets without additional carbon additive, and with .6‐wt% graphite or .6‐wt% diamond additives, were .453, and .517, or .525 μm, respectively.

Biochar Synthesis from Mineral- and Ash-Rich Waste Biomass, Part 1: Investigation of Thermal Decomposition Mechanism during Slow Pyrolysis.

Synthesizing biochar from mineral- and ash-rich waste biomass (MWB), a by-product of human activities in urban areas, can result in renewable and versatile multi-functional materials, which can also cater to the need of solid waste management. Hybridizing biochar with minerals, silicates, and metals is widely investigated to improve parent functionalities. MWB intrinsically possesses such foreign materials. The pyrolysis of such MWB is kinetically complex and requires detailed investigation. Using TGA-FTIR, this study investigates and compares the kinetics and decomposition mechanism during pyrolysis of three types of MWB: (i) mineral-rich banana peduncle (BP), (ii) ash-rich sewage sludge (SS), and (iii) mineral and ash-rich anaerobic digestate (AD). The results show that the pyrolysis of BP, SS, and AD is exothermic, catalyzed by its mineral content, with heat of pyrolysis 5480, 4066, and 1286 kJ/kg, respectively. The pyrolysis favors char formation kinetics mainly releasing CO2 and H2O. The secondary tar reactions initiate from ≈318 °C (BP), 481 °C (SS), and 376 °C (AD). Moreover, negative apparent activation energies are intrinsic to their kinetics after 313 °C (BP), 448 °C (SS), and 339 °C (AD). The results can support in tailoring and controlling sustainable biochar synthesis from slow pyrolysis of MWB.

Highly efficient degradation of phenolic compounds by Fe(II)-activated dual oxidant (persulfate/calcium peroxide) system

This study demonstrates the feasibility, reaction mechanisms, and potential of practical applications of a dual oxidant (DuOx) system comprising calcium peroxide (CP) and persulfate (PS) catalyzed using Fe(II) [PS/CP/Fe(II)]. The DuOx system was superior in phenol degradation to single oxidant systems, i.e., PS/Fe(II) or CP/Fe(II), with 95.5% phenol removal under an optimum condition of a phenol/PS/CP/Fe(II) molar ratio of 1/1/5/6 ([Phenol]0=0.5 mM). Based on scavenger studies and electron spin resonance (ESR) spectroscopy, the phenol removal in the DuOx system was barrierless, with negative activation energy assisted by robust reactive species. The phenol degradation results in the presence of methanol, t-butanol, l-histidine, and NaN3. The ESR spectroscopy indicates that phenol degradation is attributed dominantly to 1O2 generated by recombining O2•− and radicals, such as hydroxyl (HO•) and sulfate (SO4•−). The performance of the DuOx system was highly efficient in pH 3–11, up to 10 mM Cl−, SO42−, or NO3−, and up to 50 mg/L humic acids but was strongly suppressed by more than 10 mM HCO3− and H2PO4−. In addition, the DuOx system was efficient in phenol removal in natural groundwater as well as removing and mineralizing other phenolic compounds (PCs) such as bisphenol A, chlorophenol, dichlorophenol, trichlorophenol, and nitrophenol. These results provide insights into the reactions induced by the DuOx system and confirm its applicability of in situ chemical oxidation in refractory organic pollutants.

Phase Transformation in Quartz at Elevated Temperatures

Natural quartz is a raw material in Si production. During heating, it will go through a series of phase transformations before melting in the furnace, from quartz to an intermediate amorphous phase and further to cristobalite. The formation of the intermediate amorphous and cristobalite phases may affect the permeability and reaction rate in the furnace. Hence, it is crucial to know the quartz behavior. In this regard, the phase transformations in four different quartz types, named A, D, F, G, were investigated in the temperature range of 1500-1650°C. A numerical model was introduced to describe the kinetics of the phase transformations. The kinetic constant of k1 indicated the first-order reaction from quartz to the intermediate amorphous phase, which is in the range of 0.012-0.168min-1; while the rate constant k2 corresponds with the second-order reaction from the intermediate amorphous phase to cristobalite, is calculated in the range of 0.00036-0.0097min-1. The activation energies corresponding to k1 and k2 were calculated as positive(199 to 364kJ/mol) and negative values (-108 to -448kJ/mol), respectively, which indicated the different temperature dependence of these two reaction steps. The negative activation energy in the reaction from amorphous phase to cristobalite signifies that the amorphous phase is more stable at higher temperatures. The effect of impurities and grain size on the kinetics were also discussed in the article. It is verified that the numerical model was reliable to predict the reaction rates and activation energies in the quartz phase transformation at a temperature lower than 1600°C since there was already a soften/melting of quartz at 1650°C in some types.

Meta-analysis of viscosity of aqueous deep eutectic solvents and their components

Deep eutectic solvents (DES) formed by quaternary ammonium salts and hydrogen bond donors are a promising green alternative to organic solvents. Their high viscosity at ambient temperatures can limit biocatalytic applications and therefore requires fine-tuning by adjusting water content and temperature. Here, we performed a meta-analysis of the impact of water content and temperature on the viscosities of four deep eutectic solvents (glyceline, reline, N,N-diethylethanol ammonium chloride-glycerol, N,N-diethylethanol ammonium chloride-ethylene glycol), their components (choline chloride, urea, glycerol, ethylene glycol), methanol, and pure water. We analyzed the viscosity data by an automated workflow, using Arrhenius and Vogel–Fulcher–Tammann–Hesse models. The consistency and completeness of experimental data and metadata was used as an essential criterion of data quality. We found that viscosities were reported for different temperature ranges, half the time without specifying a method of desiccation, and in almost half of the reports without specifying experimental errors. We found that the viscosity of the pure components varied widely, but that all aqueous mixtures (except for reline) have similar excess activation energy of viscous flow {E}^{excess}_{eta}= 3–5 kJ/mol, whereas reline had a negative excess activation energy ({E}^{excess}_{eta}= − 19 kJ/mol). The data and workflows used are accessible at https://doi.org/10.15490/FAIRDOMHUB.1.STUDY.767.1.

On the Physical Meaning of Single-Value Activation Energies for BTI in Si and SiC MOSFET Devices

Bias temperature instability (BTI) has been shown to be the collective response of an ensemble of defects to a gate voltage signal. Each defect can be described using first-order reactions for either charge trapping or defect creation with a broad distribution of time constants. Consequently, the temperature dependence of BTI has to be determined by a distribution of activation energies reflecting the variety of available defects. Commonly employed single-value apparent activation energy-based modeling of BTI can therefore be at best a very rough approximation and its apparent activation energy is consequently a model parameter without physical justification. This study investigates the impact of measurement parameters, such as the extraction point in the drift curve and the recovery time, on these apparent activation energies that can be extracted either by threshold voltage shift increase (vertical method) or by stress time acceleration (horizontal method). The framework of activation energy maps enables an analytical mathematical approach to calculate the dependence of extracted apparent activation energies on the measurement parameters. A comparison between Si and SiC MOSFET devices is employed, and nonphysical negative apparent activation energies are explained. Since apparent activation energies have been repeatedly employed to justify physics-based descriptions of BTI, their physical (in)significance is discussed.

Undercooled phase behind the glass phase with superheated medium-range order above glass transition temperature

Rapidly quenched glass formers are amorphous and transformed into glass phases by relaxing enthalpy during the first heating. Two liquids give rise, at first, to an intermediate Phase 3 below T3 < Tg respecting the entropy constraints and then, the enthalpy increases towards that of the glass phase up to Tg. The negative activation energy shows that Phase 3 is hidden behind the glassy phase acting as an intermediate invasive phase during the second cooling. Phase 3 carries a medium-range order above Tg which can be superheated above the melting temperature up to Tn+. The two-liquid state model predicts the thermodynamic properties as well as the relaxation times from liquids 1 to 2. The configuron model is successfully applied to 54 glasses explaining the transitions by percolation and an ‘ordered’ fraction equal to the critical threshold Φc = 0.15 ± 0.01 from Tg to Tn+.

Effect of Li2O on melt crystallization of CaO–SiO2–CaF2 based glasses

This paper presents the effects of Li2O on the kinetics and structural aspects of the Cuspidine (Ca4Si2F2O7) crystallization behavior of CaO–SiO2–CaF2 glass (basicity 1.7). In order to elucidate the crystallization characteristics during differential scanning calorimetry (DSC) measurements, the kinetic parameters have been determined using the Johnson-Mehl-Avrami-Kolmogorov (JMAK) equation. The crystallization rate constant and negative activation energy thus calculated indicated that the limiting step of crystallization was nucleation. Also, Raman spectroscopy and Solid-state MAS NMR spectroscopy analyses indicated that lithium could interact with fluorine, thereby disturbing the interaction between calcium and fluorine. This retards Cuspidine nucleation at the initial stage of melt crystallization. These findings on CaO–SiO2–CaF2-based glass lubricants can be used to optimize essential properties such as viscosity and crystallization temperature during continuous casting of steels.

Effect of Noble Metal Addition on the Disorder Dynamics of Ni3Al by Means of Monte Carlo Simulation

In this work, the effect of the addition of noble metals on the order–order disorder process of the L12 structure corresponding to the intermetallic Ni3Al is analyzed. Stoichiometric, nonstoichiometric, and quasi-binary compositions doped with noble metals such as Ag, Au, Pd, and Pt (1 at%) were analyzed. It was observed that depending on the composition, there is a modification in the activation energies calculated from the two time constants that characterize the disorder process. The statistic of atomic jumps was typified based on the configuration of the window to be crossed and, with this, it was identified that the origin of the negative activation energy of the long disorder process is due to an increase in the corresponding energy of the AlAl-Ni jump through unnatural windows.

Deposition Behavior of Polyaniline on Carbon Nanofibers by Oxidative Chemical Vapor Deposition.

Oxidative chemical vapor deposition (oCVD) offers unique advantages as a liquid-free processing technique in synthesizing and integrating conducting polymers, including polyaniline (PANI), by enabling conformal coatings onto nanostructured substrates, like carbon nanofibers. With relatively thick nanofiber mats, the challenge is to ensure uniform coating thickness through the porous substrates. Here, the substrate temperature during oCVD is found to be a primary factor influencing PANI coating uniformity. Coating uniformity is enhanced by operating at a higher substrate temperature, where monomer adsorption is believed to be limiting relative to intrinsic reaction kinetics. Also, a higher substrate temperature leads to significantly less PANI oligomers and more PANI in the emeraldine oxidation state. A systematic study of oCVD kinetics with substrate temperature shows a reaction-limited regime at lower substrate temperatures with an activation energy of 12.0 kJ/mol, which is believed to be controlled by the self-catalyzed PANI polymerization reaction that transitions at higher substrate temperatures above 90 °C to an adsorption-limited regime as indicated by a negative activation energy of -18.8 kJ/mol. Overall, by operating within an adsorption-limited oCVD regime, more uniform oCVD PANI coatings on electrospun carbon nanofiber mats have been achieved.

Experimental study on dynamic characteristics of low temperature oxidation for Bohai offshore light oil under reservoir conditions

Air flooding has shown to be a promising method for enhanced oil recovery (EOR). The air injection into reservoir causes complex oxidation reaction, resulting in the change of the properties and components of crude oil. Most studies on thermal behavior and kinetics of crude oil were not conducted under realistic reservoir conditions, bringing difficulties to understand the suitability of air flooding for a light oil reservoir. The objectives of this research were to investigate dynamic characteristics of low temperature oxidation (LTO) of Bohai offshore light oil under realistic reservoir conditions. Thermogravimetry experiments (TG), static and dynamic oxidation experiments were used to evaluate the oxidation reaction of Bohai light oil reservoir. Besides, the thermal behavior of the light oil and alteration of oil components during LTO were comprehensively measured. The results indicated that there were three stages for light oil in LTO stage, the first and second stages of LTO showed positive activation energy but the third stage showed negative activation energy, manifesting the LTO stage might include multiple mechanisms. The LTO of light oil was promoted by pressure, and the negative temperature coefficient (NTC) stage of LTO might be controlled by medium oil component (C7-C17). Additionally, oxygen consume rate of dynamic oxidation experiment was much higher than that of static oxidation experiment, which was closer to the realistic reservoir conditions. This study can provide insights to the LTO of light oil reservoir and explore the potential of air flooding based on EOR projects.

Phase Change-Driven Negative Activation Energies in Pd/Carbon-Based/Organic Getter Hydrogenation Reactions.

The hydrogenation of 1,4-diphenylbutadiyne (DPB) blended with carbon-supported Pd (DPB-Pd/C) in the form of pellets was investigated by isothermal-isobaric experiments at 1333 Pa of H2 and in the temperature range of 291-315 K. The extracted kinetics were then used in conjunction with a complementary constant rate of H2 input experimentation to model the performance of a DPB-catalysis/support system as a function of temperature and H2 partial pressure. First-principles density functional theory (DFT) calculations were also performed to shed light on the molecular level energetics of DPB and its intermediate states. A seemingly puzzling formation of alternate positive activation energy barrier (higher reaction rate with higher temperature) and negative activation energy barrier (higher reaction rate with lower temperature) zones during the hydrogenation process was discovered. However, this observed phenomenon can be logically explained in terms of the associated phase changes and H2 transport in the material. This work provides a good illustration of a rarely encountered chemical process with a negative activation energy barrier.

Temperature‐dependent kinetics of the simplest Criegee intermediate reaction with dimethyl sulfoxide

AbstractCriegee intermediates are thought to play roles in atmospheric chemistry, including OH radical formation, oxidation of SO2, NO2, etc. CH2OO is the simplest Criegee intermediate, of which the reactivity has been a hot topic. Here we investigated the kinetics of CH2OO reaction with dimethyl sulfoxide (DMSO) under 278–349 K and 10–150 Torr. DMSO is an important species formed in the oxidation of dimethyl sulfide in the biogenic sulfur cycle. The concentration of CH2OO was monitored in real‐time via its mid‐infrared absorption band at about 1,286 cm−1 (Q branch of the ν4 band) with a high‐resolution quantum cascade laser spectrometer. The 298 K bimolecular rate coefficient was determined to be k298 = (2.3 ± 0.3) × 10−12 cm3/s at 30 Torr with an Arrhenius activation energy of −3.9 ± 0.2 kcal/mol and a weak pressure dependence for pressures higher than 30 Torr (k298 = (2.8 ± 0.3) × 10−12 cm3/s at 100 Torr). The reaction is speculated to undergo a five‐membered ring intermediate, analogous to that of CH2OO with SO2. The negative activation energy indicates that the rate‐determining transition state is submerged. The magnitude of the reaction rate coefficient lies in between those of CH2OO reactions with (CH3)2CO and with SO2.